1. Field of the Invention
The present invention relates to a control unit suitably applied for controlling voltage or current in electrical equipment.
2. Description of the Related Art
Recently, convenience and efficiency of electronics technology has been highly evaluated, which accelerates electronic equipment technology represented by IT (information technology) and AV (audio visual) technology to be widely used on a global scale. On the other hand, the importance of the protection of global environment and limited earth resources has been pointed out. Thus, the development of an energy saving technology for such equipment has been desired greatly.
For example, efficiency in power supply of electronic equipment has been continuously improved, and some switching supply has achieved an efficiency of 90% or more. In reality, however, the power supply having low efficiency in view of cost or noise reduction is still used in many cases.
Also, even in the power supply having a high efficiency, the efficiency is affected by fluctuation of input power source voltage, component variation, and change of load current, and is decreased considerably in a low power operation, for example.
Although power supply efficiency is generally designed to be high at the rated load (power) of equipment, operation power continuously fluctuates in actual equipment and the efficiency thereof changes at the same time. In a television receiver, for example, the operation power thereof changes considerably according to an audio output level or a luminance level of the screen thereof. In other words, there exists an optimum input voltage for load current.
Further, the power supply efficiency becomes lower than a specified value thereof in an actual operation due to an effect of voltage fluctuation in a commercial power source. This may occur in any of switching regulators and series regulators.
For example, generally, a transformer has a no-load loss in an unloaded condition, and therefore the efficiency thereof is minimized in the unloaded condition and then increases along with the increase of a load current. However, a load loss is generated at the rate of the square of the load current and thus the load loss becomes a main factor of the entire loss, thereby decreasing the efficiency when the current exceeds a certain range.
In an actual transformerless power supply, one terminal of a commercial alternate current (AC) power source of 100 V, for example, is connected to one input terminal of a rectifying circuit composed of a diode bridge via a capacitor, and the other terminal of the commercial power source is connected to the other input terminal of the rectifying circuit. A zener diode for a constant voltage and a smoothing capacitor are connected in parallel between one and the other output terminals of the rectifying circuit.
Such a tranformerless power supply rectifies the commercial power source voltage directly and then uses a zener diode composing a regulator to provide a stable direct current (DC) voltage across the output terminals.
Here, the capacitor works for decreasing the voltage in advance and reducing the load of the zener diode composing the regulator.
A capacitor is frequently used for a small power. This is because a voltage drop by the capacitor may not cause a power loss, since the phase of current is shifted from that of voltage, and the capacitors are utilized for a power supply for standby power and the like, for example. In this circuit, however, the rectified output fluctuates according to load change and the like, and thus the circuit is generally configured to be optimized for the maximum load and to cause a power loss in the regulator at a light load so as to provide a stable voltage.
Also, the voltage drop across the capacitor changes considerably depending on a frequency or load current fluctuation. Therefore, the capacitor may not be used in equipment in which load current and load fluctuation are large, and the use thereof is currently limited to a micro-power application with a standby power of about several tens of milliwatts.
Also, in the transformerless power supply, it is possible to connect another predetermined capacitor to the capacitor in parallel using a relay or the like to increase the power supply when an operation accompanying a large power consumption is performed. However, a plurality of capacitors may need to be switched to cover a wide load range, although, in principle, it is possible to switch the plurality of capacitors with a relay or the like.
However, the power supply with switched capacitors may be slow in response in addition to requiring space and cost, and noise may be generated in the switching. Further, the capacitance may not be continuously changed in the power supply with switched capacitors, and the durability is low, and therefore it may not be put into practical use. Accordingly, it is desirable to have a device which can change the capacitance thereof continuously according to load change.
As a capacitor, the capacitance of which is electrically controllable, a varicap utilizing a capacitance across diode terminals is used for an application in a high frequency circuit, however, the varicap may not be used alone for power control because of a small capacitance value and a low withstand voltage.
Also, recently, a plurality of variable capacitors utilizing MEMS (micro-electromechanical system) is proposed. However, such capacitors may need to be used with a high frequency signal.
Generally, the capacitance of a capacitor is determined by a dielectric constant, an electrode area, and a distance between electrodes. Therefore, at least one factor among them may need to be controlled for controlling the capacitance. A method for controlling capacitance actually proposed using the MEMS is to change the inter-electrode distance or the facing electrode area by displacing the electrodes.
Japanese Unexamined Patent Application Publication No. S62-259417, for example, discloses an example of changing a dielectric constant of a ceramic capacitor by applying 50 V to change the capacitance thereof by 70%, and proposes an application thereof for making a cutoff frequency of a filter circuit or an oscillation frequency of an oscillator circuit with a time constant variable, for example.